The present invention relates to an immersion heater for heating electroplating solutions wherein a heater tube is encased in a material having a low surface coefficient of friction and a high resistance to heat and attack by acid or alkaline solutions or vapors such as the fluorocarbon polymers, particularly polytetrafluoroethylene tubing or films. While the heater can be used for any desired purpose, it is most particularly adapted for immersion into a highly corrosive solution and/or atmosphere of an electroplating solution in a container to maintain the solution at a desired temperature during an electroplating operation.
In the design and manufacture of electrical resistance heaters for immersion in liquid baths, or immersion heaters, as they are called, it is desirable to protect the electrical heating element from any corrosive effects of the bath or the atmosphere above the bath. Likewise, it is desirable to have a high degree of flexibility to the heater tube to permit coiling and winding on supports to increase the length of heater which may be immersed in the bath. Flexible immersion heaters having an inert plastic encasing the heating element are known in the art, as for example those described in U.S. Pat. Nos. 3,674,985; 3,657,520 and 4,158,764. The first two of these mentioned patents discribe such known immersion heaters which lack the desired flexibility because of a solid linear heating element as used with a braided glass sheath covered with a coating of rubber bonded to the outer surface of the sheath. The latter of these patents, namely, U.S. Pat. No. 4,158,764 utilized a coil conductor formed from an alloy having a high electrical resistance and has an outer tubular casing of plastic material which is inert to acid or alkaline solutions. A flexible sleeve braided of fibrous glass material is slideably assembled on the coiled wire and the sleeve with coiled wire within is inserted as a subassembly into the outer tubular member to form an assembly for immersion in the liquid to be heated. A number of these prior art disclosures indicate that the plastic material protecting the resistance heater within the flexible heating tube can be polytetrafluoroethylene.
Many attempts have been made in the past to utilize a polytetrafluoroethylene outer protective coating for such immersion heaters for use in corrosive baths and/or atmospheres as found in electroplating. Initially, such heaters were found to be defective in that that the polytetrafluoroethylene (PTFE) was extremely difficult to extrude or otherwise be formed into a continuous film so as to adequately protect the resistance heating means within the heater tube. Pinholes in the PTFE allowed corrosive solutions to pass through the PTFE and relatively rapidly corrode and ruin such immersion heaters. Although today's PTFE tubing is more void free than that produced as little as five years ago, it is still defective in a number of instances due to poor tubing formation.